Commercial feed appears to be a source of Salmonella contamination in commercial swine production units, according to a paper in the November 2010 issue of the journal Applied and Environmental Microbiology. Moreover, nearly half of isolates found in pigs were multidrug resistant. The findings suggest that pork could be a source of human infection. They also strongly question the conventional wisdom that processed feed is not a source of contamination. Heat treatment during processing has been thought to kill any bacterial contaminants.

The research team, led by Wondwossen A. Gebreyes of the College of Veterinary Medicine, The Ohio State University, tested samples collected from feed bins prior to exposure to the barn environment, as well as fecal samples and environmental samples from the barns. They found contaminated feed in eight of 36 barns tested, with a sample prevalence of 3.6%. These isolates fell into five different genotypes. In four of the five cases, they found that fecal samples they tested from a given barn and time point matched the feed samples from the same barn and time period, suggesting that the feed was indeed the contamination source.

“These genotypic clusters also shared similar antimicrobial resistance profiles and serogroups,” says Gebreyes. That provides additional support for both the genotypic findings, confirming the hypothesis that the contamination originated in the feed.

Gebreyes says that the source of feed contamination is most likely the feed ingredients, but that feed could be contaminated via handling, after processing.

“Although we cannot ascertain 100% that the feed was the source of contamination that was transmitted to the fecal samples, the findings strongly imply that fact,” says Gebreyes. “The other alternative is that the feed was contaminated after it was introduced into the barn. Regardless, the findings strongly imply that salmonella can be maintained and easily disseminated in a population of food animals.”